Multi-layer systems have been utilized to coat automobiles for a number of years, but the early development of these systems necessarily employed organic solvents. As environmental regulations became more stringent organic-borne systems became less desirable. The recent research emphasis in the area of multi-layer systems, especially basecoat systems, has focused on the development of water-borne systems for multi-layer coatings.
The known aqueous basecoats use anionically stabilized resins.
The U.S. Pat. No. 4,403,003 discloses a basecoat which comprises a dispersion of crosslinked polymer microparticles in an aqueous medium. But these microgels have the deficiencies of being ardorous to make and difficult to stabilize.
The U.S. Pat. No. 4,719,132 discloses a process for producing multi-layered coated articles. The film forming composition is an aqueous anionic polyurethane dispersion.
These basecoats have several drawbacks which relate to the preferred stabilization method. The preferred amines used to salt the acid groups interfere with the cure of strong acid catalyzed melamine clearcoats. This results in softer clearcoat and/or wrinkling in the clearcoat. Addition of acids, such as those used to produce lower temperature curing melamine systems, results in flocculation or coagulation of the anionically stabilized resin. Exposure of the anionically stabilized basecoats to polyvalent cations, such as those be released from corrosion of steel, also result in flocculation or coagulation of the anionically stabilized resin.
An object of the present invention is to provide an aqueous coating composition which is stable in the presence of acids and bases and which does not interfere with the cure of crosslinkers like melamine.
Another object is to provide a basecoat with a good appearance (metallic effect).
Another object is to provide a multilayer coating system.